Data on water quality of Lake Shinji and Lake Nakaumi were obtained at one hour intervals using an automatic water quality monitoring device provided by the Ministry of Land, Infrastructure and Transport, Government of Japan. Out of the various parameters measured, the data on water temperature, salinity and chlorophyll-a were validated by comparing with the field data obtained through in situ measurements during 1999-2001. The difference between the automatically measured sensor data and the field data was estimated by comparing these data without considering the vertical variations. The results show that the difference between the two datasets of water temperature is 0.0±0.4℃ in Lake Shinji and -0.2±0.2℃ in Lake Nakaumi, that of salinity is 0.2±0.1 psu in Lake Shinji and 0.0±0.3 psu in Lake Nakaumi and that of chlorophyll-a is 0.3±2.1 mg/m^3 in Lake Nakaumi(excluding the middle layer value).